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Diffstat (limited to 'doc/man3/SSL_CTX_set_client_hello_cb.pod')
-rw-r--r-- | doc/man3/SSL_CTX_set_client_hello_cb.pod | 130 |
1 files changed, 130 insertions, 0 deletions
diff --git a/doc/man3/SSL_CTX_set_client_hello_cb.pod b/doc/man3/SSL_CTX_set_client_hello_cb.pod new file mode 100644 index 000000000000..6824b5b8d1a4 --- /dev/null +++ b/doc/man3/SSL_CTX_set_client_hello_cb.pod @@ -0,0 +1,130 @@ +=pod + +=head1 NAME + +SSL_CTX_set_client_hello_cb, SSL_client_hello_cb_fn, SSL_client_hello_isv2, SSL_client_hello_get0_legacy_version, SSL_client_hello_get0_random, SSL_client_hello_get0_session_id, SSL_client_hello_get0_ciphers, SSL_client_hello_get0_compression_methods, SSL_client_hello_get1_extensions_present, SSL_client_hello_get0_ext - callback functions for early server-side ClientHello processing + +=head1 SYNOPSIS + + typedef int (*SSL_client_hello_cb_fn)(SSL *s, int *al, void *arg); + void SSL_CTX_set_client_hello_cb(SSL_CTX *c, SSL_client_hello_cb_fn *f, + void *arg); + int SSL_client_hello_isv2(SSL *s); + unsigned int SSL_client_hello_get0_legacy_version(SSL *s); + size_t SSL_client_hello_get0_random(SSL *s, const unsigned char **out); + size_t SSL_client_hello_get0_session_id(SSL *s, const unsigned char **out); + size_t SSL_client_hello_get0_ciphers(SSL *s, const unsigned char **out); + size_t SSL_client_hello_get0_compression_methods(SSL *s, + const unsigned char **out); + int SSL_client_hello_get1_extensions_present(SSL *s, int **out, + size_t *outlen); + int SSL_client_hello_get0_ext(SSL *s, int type, const unsigned char **out, + size_t *outlen); + +=head1 DESCRIPTION + +SSL_CTX_set_client_hello_cb() sets the callback function, which is automatically +called during the early stages of ClientHello processing on the server. +The argument supplied when setting the callback is passed back to the +callback at runtime. A callback that returns failure (0) will cause the +connection to terminate, and callbacks returning failure should indicate +what alert value is to be sent in the B<al> parameter. A callback may +also return a negative value to suspend the handshake, and the handshake +function will return immediately. L<SSL_get_error(3)> will return +SSL_ERROR_WANT_CLIENT_HELLO_CB to indicate that the handshake was suspended. +It is the job of the ClientHello callback to store information about the state +of the last call if needed to continue. On the next call into the handshake +function, the ClientHello callback will be called again, and, if it returns +success, normal handshake processing will continue from that point. + +SSL_client_hello_isv2() indicates whether the ClientHello was carried in a +SSLv2 record and is in the SSLv2 format. The SSLv2 format has substantial +differences from the normal SSLv3 format, including using three bytes per +cipher suite, and not allowing extensions. Additionally, the SSLv2 format +'challenge' field is exposed via SSL_client_hello_get0_random(), padded to +SSL3_RANDOM_SIZE bytes with zeros if needed. For SSLv2 format ClientHellos, +SSL_client_hello_get0_compression_methods() returns a dummy list that only includes +the null compression method, since the SSLv2 format does not include a +mechanism by which to negotiate compression. + +SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(), +SSL_client_hello_get0_ciphers(), and +SSL_client_hello_get0_compression_methods() provide access to the corresponding +ClientHello fields, returning the field length and optionally setting an out +pointer to the octets of that field. + +Similarly, SSL_client_hello_get0_ext() provides access to individual extensions +from the ClientHello on a per-extension basis. For the provided wire +protocol extension type value, the extension value and length are returned +in the output parameters (if present). + +SSL_client_hello_get1_extensions_present() can be used prior to +SSL_client_hello_get0_ext(), to determine which extensions are present in the +ClientHello before querying for them. The B<out> and B<outlen> parameters are +both required, and on success the caller must release the storage allocated for +B<*out> using OPENSSL_free(). The contents of B<*out> is an array of integers +holding the numerical value of the TLS extension types in the order they appear +in the ClientHello. B<*outlen> contains the number of elements in the array. + +=head1 NOTES + +The ClientHello callback provides a vast window of possibilities for application +code to affect the TLS handshake. A primary use of the callback is to +allow the server to examine the server name indication extension provided +by the client in order to select an appropriate certificate to present, +and make other configuration adjustments relevant to that server name +and its configuration. Such configuration changes can include swapping out +the associated SSL_CTX pointer, modifying the server's list of permitted TLS +versions, changing the server's cipher list in response to the client's +cipher list, etc. + +It is also recommended that applications utilize a ClientHello callback and +not use a servername callback, in order to avoid unexpected behavior that +occurs due to the relative order of processing between things like session +resumption and the historical servername callback. + +The SSL_client_hello_* family of functions may only be called from code executing +within a ClientHello callback. + +=head1 RETURN VALUES + +The application's supplied ClientHello callback returns +SSL_CLIENT_HELLO_SUCCESS on success, SSL_CLIENT_HELLO_ERROR on failure, and +SSL_CLIENT_HELLO_RETRY to suspend processing. + +SSL_client_hello_isv2() returns 1 for SSLv2-format ClientHellos and 0 otherwise. + +SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(), +SSL_client_hello_get0_ciphers(), and +SSL_client_hello_get0_compression_methods() return the length of the +corresponding ClientHello fields. If zero is returned, the output pointer +should not be assumed to be valid. + +SSL_client_hello_get0_ext() returns 1 if the extension of type 'type' is present, and +0 otherwise. + +SSL_client_hello_get1_extensions_present() returns 1 on success and 0 on failure. + +=head1 SEE ALSO + +L<ssl(7)>, L<SSL_CTX_set_tlsext_servername_callback(3)>, +L<SSL_bytes_to_cipher_list> + +=head1 HISTORY + +The SSL ClientHello callback, SSL_client_hello_isv2(), +SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(), +SSL_client_hello_get0_ciphers(), SSL_client_hello_get0_compression_methods(), +SSL_client_hello_get0_ext(), and SSL_client_hello_get1_extensions_present() +were added in OpenSSL 1.1.1. + +=head1 COPYRIGHT + +Copyright 2017 The OpenSSL Project Authors. All Rights Reserved. + +Licensed under the OpenSSL license (the "License"). You may not use +this file except in compliance with the License. You can obtain a copy +in the file LICENSE in the source distribution or at +L<https://www.openssl.org/source/license.html>. + +=cut |